CHEMISTRY OF BREATHING. 335 



The white precipitate is carbonate of lime, 

 an earth formed by the union of carbonic acid 

 gas with lime. From analyses already given 

 of the atmospheric composition, carbonic acid 

 has been found to be invariably present in air, 

 and therefore it might be said this precipitate 

 indicates nothing more than what might have 

 been expected. And it is true that lime-water, 

 after being exposed for any length of time to 

 the air, is decomposed, and carbonate of lime 

 falls to the bottom of the vessel But in the 

 simple experiments above mentioned this diffi- 

 culty disappears, for it will be found that the 

 bellows must be moved all day long to get a 

 sensible precipitation, whereas a single expira- 

 tion of air from the lungs will instantly render 

 the fluid turbid. Although, therefore, it is 

 certain that a minute portion of carbonic acid 

 exists in all air, it is, on the other hand, equally 

 certain that there is an enormous disproportion 

 in the quantities contained in ordinary and in 

 expired air. In the one the amount is merely 

 fractional ; in the other it is present to a large 

 per centage. The air which has done duty in 

 the lungs receives a large amount of carbonic acid 

 gas as it leaves them. 



Let us now enter upon another range of 

 thought. In cases of disease it is very often 

 considered necessary to open a vein, and to 



